1. Field of the Invention
This invention generally relates to integrated circuit (IC) testing and, more particularly, to a liquid metal socket IC test fixture and a liquid metal socket test method.
2. Description of the Related Art
Because of the critical applications in which ICs are used, it is typical for an IC manufacturer to test parts before they are distributed. Conventional ceramic packages with formed attachment leads make the testing process convenient, and the part may be inserted and removed an indefinite number times without damage. However, many ICs are sold as IC packages with ball grid array (BGA) or bump interfaces. These BGA interfaces can handle only a limited number of insertions before damage occurs. As a result, many of these IC packages with BGA interfaces are damaged during the testing process. IC packages with damaged BGA interfaces must have the BGA connections removed, and new BGA connections attached. The BGA reattachment process is expensive and can result in the irreparable damage to some IC packages. Therefore, IC manufacturers are constantly seeking methods of testing an IC package that are less detrimental to the BGA interfaces.
FIG. 1 is a depiction of an IC package test fixture using a pogo pin socket (prior art). The fixture uses free-floating gold plated pins held captive in a plastic dielectric. The frame is typically a combination of plastic and aluminum. The IC is held in place by a lid that either is a hinge and clasp, or two clasps. Pressure is applied through a torque screw that causes the pogo pins to connect with the BGA connectors. Such a fixture has the advantage of removable sockets that are not soldered in place. The fixture itself is able to withstand a high number of insertions without damage to the pins. Such a fixture is adequate for functional testing or simple validation. However, the lead inductance associated with the pogo pins degrades signals at high frequencies. Likewise, impedance mismatch between the BGA interface and the pogo pins affects signal integrity. The socket temperature coefficient is different from the IC package, requiring a technician to readjust socket connections under different temperature conditions. Also, the test fixture pins can deform the balls. Further, the fixture can""t be used with a non-balled IC package.
FIG. 2 is a depiction of an IC package test fixture using a fuzz button socket (prior art). The so-called fuzz buttons are a material that closely resembles very small steel wool pads placed on a circuit board. The frame is typically a combination of plastic and aluminum. The IC is held in place by a lid that either is a hinge and clasp, or two clasps. Pressure is applied with a leaf spring tensioner.
Compared to pogo pins, the fuzz buttons have less inductance, they mate to the balls better, and they inflect fewer deformities and damage to the BGA balls. However, fuzz buttons have a very limited lifespan. The material deforms and must be replaced after only three or four insertions. Further, a fuzz button test fixture cannot be used with a non-balled IC package.
FIG. 3 is a depiction of an IC test fixture using a z-axis gold pin socket (prior art). Each z-axis pin includes multiple wire threads embedded in an elastomer type material. The frame is typically a combination of plastic and aluminum. The IC is held in place by a lid that either is a hinge and clasp, or two clasps. Pressure is applied with a leaf spring tensioner.
Advantageously, the z-axis pin is a removable socket, not soldered into place, that is easy to replace. The z-axis pin has less inductance, compared to pogo pins and, as a result, is capable of interfacing higher frequency signals. However, the z-axis pins can only be used for a very limited number of insertions and can""t be used with a non-balled IC package.
FIG. 4 is a depiction of an IC test fixture using a diamond dust, or particle interconnect socket (prior art). Gold plated diamond dust is bonded directly to the circuit board, resembling patches of sand paper. This technique was originally developed for IC flat packs and J-lead style packages. The frame is typically a combination of plastic and aluminum. The IC is held in place by a lid that either is a hinge and clasp, or two clasps. Pressure is applied through a torque screw.
Advantageously, the diamond dust socket has a minimal inductance, promoting a wide bandwidth and high frequency capability. Also, the diamond dust socket is capable of a high number of insertions. However, the lack of uniformity in the diamond deposition, along with ball coplanar non-uniformities mandates that a higher pressure be exerted on the IC package during test. As a result, the balls can be damaged or deformed. Further, the diamond dust socket can""t be used with a non-balled IC package.
It would be advantageous if an IC package test fixture could be developed with low-inductance sockets that were capable of many insertions, with damaging the IC package BGA connectors.
It would be advantageous if the above-mentioned socket could be used to test an IC package regardless of whether the IC package had a BGA interface.
The present invention uses a liquid metal compound deposited in small wells on a circuit board to form a connection with a BGA package. A gravity-tension frame is used, typically made of plastic. However, no tensioner is needed, as gravity is sufficient to ensure a good contact between the IC package and the liquid metal compound.
A test fixture using the present invention liquid metal sockets has a performance that closely resembles that of a soldered-down part. The fixture is capable of a very high number of insertions, as the socket doesn""t wear out. The liquid metal socket can be used with balled or non-balled IC packages, and they do not deform or damage BGA balls.
Accordingly, a liquid metal socket test fixture system is provided. The system comprises a circuit board top surface and a plurality of wells formed in the circuit board top surface. A liquid metal compound forms balls in the wells. An IC package having a bottom surface with electrical contacts interfaces with the liquid metal compound in the wells. In some aspects of the system, the IC package includes solid ball grid array (BGA) connectors attached to the bottom surface electrical contacts, interfacing with the liquid metal compound. Alternately, the liquid metal compound interfaces directly to the IC package bottom surface contacts.
A gravity-tension frame overlies the circuit board top surface, with interior walls sections surrounding the wells. The frame provides support in the horizontal plane so that the IC package electrical contacts remain aligned with the liquid metal balls. Typically, the liquid metal compound is a mixture of approximately 24% indium and 76% gallium. Although other room temperature liquid metals such as mercury are also practical, but may require special handling as a hazardous material.
Additional details of the above-described liquid metal socket test fixture system, and a liquid metal socket method for electrically connecting an IC package are provided below.